Method and apparatus for clutch pressure control

ABSTRACT

A method, apparatus and system for controlling transmission clutch system output pressures is provided. A transmission control unit and a pressure control device including an electro-hydraulic valve and a pressure switch cooperate to provide self-calibrating clutch pressure control systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 12/423,239, which claims priority to and thebenefit of U.S. Provisional Patent Application Ser. No. 61/049,636,filed May 1, 2008, and claims priority to and the benefit of U.S.Provisional Patent Application Ser. No. 61/050,842, filed May 6, 2008,all of which are hereby incorporated herein by reference in theirentirety.

BACKGROUND

Friction devices, such as clutches and brakes, of an automatictransmission of a vehicle are selectively engageable and disengageableto change gear ratios or alter the speed of the vehicle. For example, toshift from one transmission gear ratio to another, one clutch isdisengaged and another clutch is engaged.

Electro-hydraulic valves are often used in automatic transmissions tocontrol engagement and disengagement of friction devices, includingtransmission clutches. To achieve an acceptable shift quality, i.e.,smoothly disengaging the off-going clutch and smoothly engaging theon-coming clutch, a typical automatic transmission electro-hydraulicvalve must output a desired clutch pressure.

Electro-hydraulic valves used in automatic transmission clutch trimsystems are available in many types, including variable bleed solenoidsand related devices. In general, all of these devices receive anelectrical input from electrical circuitry, such as an electronic orelectrical controller, and provide an amount of output pressure that isa function of the amount of electrical input. Normally, the electricalinput is electrical current. The relationship between the outputpressure and the electrical input is defined by a transfer function.

The solenoid transfer function often varies from valve to valve evenamong valves of the same type. Solenoid valve manufacturers aretherefore often challenged to minimize valve-to-valve variations in thecommand-to-output transfer function. During manufacture, valves aretypically adjusted at their “end of line” test to keep the transferfunction characteristic curve within specified limits. Such adjustmentsshift or offset the characteristic curve along the electrical input axisbut do not significantly alter the overall curve shape or slope.

While the offset adjustment is helpful in reducing valve-to-valvevariations, valve rejects still exist and add to production costs. Even“good” valves still retain some detrimental part-to-part variationwithin their specified limits.

Additionally, existing solenoid calibration systems require individualsolenoid characterization data to be captured during solenoidmanufacture and then loaded into the on-board transmission controllerduring transmission manufacture. Such systems are not usable aftertransmission manufacture; for example, when individual solenoids mayneed to be replaced in a service environment.

SUMMARY

According to one aspect of the present invention, methods forcontrolling a transmission clutch pressure are provided. In oneembodiment, a method for calibrating a clutch pressure control system ofan automatic transmission of a vehicle is provided. The method includesobtaining at least one reference output pressure value and at least onereference electrical input value related to the reference outputpressure value for a pressure control device in an operating range of anautomatic transmission of a vehicle, actuating a pressure switch coupledto the pressure control device to obtain an actual electrical inputvalue corresponding to the reference output pressure value, calculatingan offset between the reference electrical input and the actualelectrical input, and applying the offset to the at least one referenceelectrical input value.

The obtaining step may include obtaining a plurality of referencepressure values in an operating range and a plurality of referenceelectrical input values related to the reference pressure values in theoperating range, and the applying step may include selectively applyingthe offset to only certain of the reference electrical input values inthe operating range.

The applying step may include selectively applying no offset to at leastone reference electrical input value in a first portion of the operatingrange. The applying step may include selectively applying the fulloffset to at least one reference electrical input value in a secondportion of the operating range different than the first portion of theoperating range. Also, the applying step may include selectivelyapplying a proportional offset to at least one reference electricalinput value in a third portion of the operating range different than thefirst and second portions of the operating range. The first portion ofthe operating range may be above an upper reference output pressurevalue. The second portion of the operating range may be below a lowerreference output pressure value. The third portion of the operatingrange may be between the upper reference output pressure value and thelower reference output pressure value.

The obtaining step may include obtaining a first reference outputpressure value located near an upper end of an operating range and atleast one reference electrical input value related to the firstreference output pressure value, obtaining a second reference outputpressure value located near a lower end of an operating range and atleast one reference electrical input value related to the secondreference output pressure value, the actuating step may includeactuating the pressure switch in a first position to obtain a firstactual electrical input value corresponding to the first referenceoutput pressure value and actuating the pressure switch in a secondposition to obtain a second actual electrical input value correspondingto the second reference output pressure value, the calculating step mayinclude calculating a first offset between the first referenceelectrical input and the first actual electrical input and calculating asecond offset between the second reference electrical input and thesecond actual electrical input, and the applying step may includeapplying the first and second offsets to the at least one referenceelectrical input value.

The method may be repeated at a plurality of different operatingtemperatures of the pressure control device. The obtaining an actualelectrical input may include receiving at a controller an electricalsignal from the pressure switch. The method may include storing the atleast one reference electrical input values in a computer-readablemedium coupled to a transmission control module.

According to another aspect of the present invention, an apparatus forcontrolling a transmission clutch pressure is provided, including ahydraulic fluid supply, an electro-hydraulic pressure control valvecoupled to the hydraulic fluid supply, a pressure switch coupled to theelectro-hydraulic pressure control valve, and a controller configured tosend electrical inputs to the electro-hydraulic pressure control valve,monitor the pressure switch, compare at least one selected electricalinput to at least one reference electrical input, and selectively modifythe at least one reference electrical input.

The electro-hydraulic pressure control valve may include a solenoid anda pressure control valve coupled to the solenoid. The pressure controlvalve may include an axially translatable spool, a first land, a secondland longitudinally spaced from the first land to define a first fluidchamber therebetween, and a return spring. The pressure switch may be influid communication with the first fluid chamber, and the return springmay be configured to prevent spool movement until a desired solenoidpressure is attained.

The spool may be configured to move when the desired solenoid pressureis attained, movement of the spool may actuate the pressure switch, andactuation of the pressure switch may signal the controller to record theamount of electrical input required to achieve the desired pressure.

The pressure control valve may include a third land spaced between thereturn spring and the second land. The third land may have adifferential area. The differential area may be configured to receivecontrol pressure applied thereto, such that when control pressure isapplied to the differential area of the third land, the return springand the differential area cooperate to bias the valve in an “off”position.

The spool may be configured to move from the biased position when asecond desired solenoid pressure is attained. Movement of the spool mayactivate the pressure switch, and activation of the pressure switch maysignal the controller to record a second amount of electrical inputrequired to achieve the second desired pressure.

The spool may be configured to move when a desired solenoid pressure isattained. Movement of the spool may toggle the pressure switch betweenfirst state and a second state, and a change from the first state to thesecond state of the pressure switch may signal the controller to recordthe amount of electrical input required to achieve the desired pressure.

The reference electrical input and/or the selectively modified referenceelectrical input may be stored in a storage medium accessible by thecontroller, such as a look-up table, database, or similar datastructure.

Patentable subject matter may include one or more features orcombinations of features shown or described anywhere in this disclosureincluding the written description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description refers to the following figures in which:

FIG. 1 is a block diagram of a driveline of a vehicle equipped with anautomatic transmission and a clutch pressure control in accordance withthe present invention;

FIG. 2 is a flow chart illustrating functional routines of an automatictransmission clutch pressure control process executable by atransmission controller or other control unit;

FIG. 3 is a flow chart of functional operations performable by atransmission controller or other control unit to control clutchpressure;

FIG. 4 is a graph illustrating aspects of a first, “single point”pressure control method;

FIG. 5 is a graph illustrating aspects of a second, “dual point”pressure control method;

FIG. 6 is a schematic of a pressure control apparatus usable to executesteps of a single or dual point solenoid pressure control method, shownin a first characterization position;

FIG. 7 is a schematic of a pressure control apparatus usable to executesteps of a dual point solenoid pressure control method, shown in asecond characterization position;

FIG. 8 is a graph illustrating aspects of a third, “modified singlepoint” pressure control method;

FIGS. 9-11 are graphs illustrating individual steps of the secondpressure control method;

FIG. 12 is a schematic of a pressure control apparatus usable to executesteps of the third pressure control method; shown in an “off” position;

FIG. 13 is a schematic of the pressure control apparatus of FIG. 12,shown in a “trim” position; and

FIG. 14 is a schematic of the pressure control apparatus of FIG. 12,shown in an “on” position.

In general, like structural elements on different figures refer toidentical or functionally similar structural elements although referencenumbers may be omitted from certain views of the drawings forsimplicity.

DETAILED DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are described with reference to certainillustrative embodiments shown in the accompanying drawings anddescribed herein. While the present invention is described withreference to the illustrative embodiments, it should be understood thatthe present invention as claimed is not limited to the disclosedembodiments.

Aspects of the present invention are directed to improving the abilityof the transmission to compensate for variations in the solenoid valvetransfer function from valve to valve. The illustrated embodiments areparticularly directed to improving pressure control after installationof the solenoids in an automobile transmission assembly or otherelectro-hydraulic control system. Such methods may be conducted duringtransmission manufacture or assembly, or during operation of thetransmission in real time. Such improvements may be expected to improvetransmission shift quality by providing calibration during transmissionoperation, thereby increasing customer satisfaction. Such improvementsmay also lower the cost of the electro-hydraulic valves since a greatervalve-to-valve variation can be tolerated.

Further, solenoid performance varies according to changes in thetemperature of the transmission. The disclosed improvements maytherefore result in improvements to temperature compensation tables whenapplied during operation of the transmission.

While the present invention is described herein in the context of anautomatic transmission of a motor vehicle, it is also applicable toother electro-hydraulic control systems in which a firstelectro-hydraulic apparatus having a lower range of possible outputpressures (such as a solenoid, which may have a pressure range of 0-100psi) is used to control another hydraulic apparatus having a higherrange of possible output pressures (such as a spool valve, which mayhave a pressure range of 0-300 psi).

Details of the present invention may be described herein with referenceto either normally high solenoids, in which pressure is output when noelectrical input is applied to the solenoid and no pressure is outputwhen electrical input is applied to the solenoid, or normally lowsolenoids, in which pressure is output when electrical input is appliedto the solenoid and no pressure is output when no electrical input isapplied to the solenoid. It will be understood by those skilled in theart that the present invention may be used to control pressure insystems using either type of solenoid, by reversing the application ofelectrical input.

In the illustrated embodiments, pressure switches, hydraulic logic andsolenoid current control are used in combination to calibrate solenoidperformance and provide pressure control. A pressure switch is activatedby movement of a spool valve to establish one or more measuredperformance points on the pressure-current (P/I) curve of the respectivesolenoid.

In one embodiment, a clutch pressure control (CPC) 34 is provided in anelectrical control 32 for an automatic transmission 14. Control 34comprises computer programming instructions or logic executable toperform one or more of the methods described herein. A microprocessor orsimilar device of electrical control unit 32 is configured to access andexecute control 34.

In general, control unit 32 controls operation of transmission 14 basedon inputs from drive unit 10, torque converter 12, transmission 14,range selector 58, and/or other inputs. Such inputs may includeelectrical and/or analog signals received from sensors, controls orother like devices associated with the vehicle components. For instance,inputs may include signals indicative of transmission input speed,driver requested torque, engine output torque, engine speed, temperatureof the hydraulic fluid, transmission output speed, turbine speed, brakeposition, gear ratio, torque converter slip, and/or other measurableparameters.

Electrical control 32 generally includes electrical circuitry configuredto process, analyze or evaluate one or more inputs and issue electricalcontrol signals as needed through one or more electrical lines orconductors. Such connections may include hard-wired and/or networkedcomponents in any suitable configuration including, for example,insulated wiring and/or wireless transmission as may be appropriate ordesired.

Electrical circuitry of control 32 includes computer circuitry such asone or more microprocessors and related elements configured to processexecutable instructions expressed in computer programming code or logic,which is stored in one or more tangible media, i.e., any suitable formof memory or storage media that is accessible or readable by theprocessor or processors. Control 32 may also include analog to digitalconverters and/or other signal processing circuitry or devices as neededto process one or more of the inputs received from the vehiclecomponents.

While shown schematically as a single block 32, it will be understood bythose skilled in the art that portions of control 32 may be implementedas separate logical or physical structures. For example, control 34 maybe physically and/or logically separated from electronic controls fortransmission 14 or electronic controls for drive unit 10. All orportions of control 34 may alternatively or in addition be executed by acontroller that is not on-board the transmission, such as an externalcontroller located at the transmission manufacturer or assembly locationbut is connectable to the transmission.

Electrical control 32 is in communication with drive unit 10 via one ormore links 48, with clutch control valves 22 via one or more links 50,with pressure switches 24 via one or more links 52, with transmission 14via one or more links 54, and with a range selector 58 via one or morelinks 56.

Drive unit 10 includes an internal combustion engine, such as aspark-ignited engine or diesel engine, an engine-electric motorcombination, or the like. Drive unit 10 is coupled to transmission 14 bya transmission input shaft 36. A fluidic torque converter 12 isgenerally interposed between drive unit 10 and transmission 14 toselectively establish a mechanical coupling. Transmission 14 is coupledto the vehicle drive wheels via an output shaft 38 in one of severalconventional ways. A transfer case 20 may be shiftable to select one ofseveral drive conditions, including various combination s of two-wheeldrive and four-wheel drive, high or low speed ranges, and the like.

Transmission 14 is an automatic transmission. Transmission 14 mayinclude a gear assembly of the type described in U.S. Pat. No. 4,070,927to Polak or another type, and may have an electro-hydraulic control ofthe type described in U.S. Patent Application Publication No.2003/0114261 to Moorman, et al. or in U.S. Pat. No. 5,601,506 to Long,et al. or another type. Transmission 14 is shiftable to selectivelyestablish one of several ranges including, for example, a neutral range,a reverse range, a drive range, and/or a plurality of manuallyselectable forward ranges.

The number of available forward ranges is determined by theconfiguration of the transmission gearsets 16 and clutches 18. Forexample, transmission 14 may have three interconnected planetarygearsets and five clutches which are controllable to provide six forwardgears. Other configurations, such as an eight-speed configuration, mayalso be used.

Operation of clutches 18 is controlled by an electro-hydraulic controlsystem including a plurality of control valves 22 and a supply ofhydraulic fluid 26. In general, each valve 22 includes a solenoid, suchas a variable bleed solenoid, on/off solenoid, or similar device. Fluidsupply 26 is operable to supply hydraulic fluid to torque converter 12via one or more passages or conduits 42 and to valves 22 via a pluralityof passages or conduits 40, 44. Pressure regulator valves 28, 30 operateto regulate fluid pressure in lines 42, 44, respectively.

Control 32 sends electrical signals to control valves 22 via the one ormore links 50, for example, in response to a shift request received fromrange selector 30. The electrical signals cause one or more of thecontrol valves 22 to adjust fluid pressure or fluid flow direction inone or more of the fluid passages connecting valves 22 and clutches 18.

The amount of electrical input required by a valve 22 to achieve adesired output pressure is generally initially set according to thevalve supplier's specifications, which are typically represented bypressure vs. electrical input (i. e., current) (“P/I”) curves, charts ortables. According to the present invention, these electrical inputrequirements are modified or “fine-tuned” for each individual valve asthe valve is actuated, through application of the disclosed methods.

In the illustrated embodiment, pressure switches 24 are operably coupledto control valves 22 to in effect render valves 22 self-calibrating inaccordance with the methods described herein.

In one embodiment, control 32 includes a microprocessor-based controller60 and CPC 34 includes a plurality of computer routines 62, 64, 66, 72,74, 76, stored in computer memory or other computer-accessible storagemedium and executable by controller 60. Pressure switches 68 senddiagnostic signals to controller 60 for processing by the routines ofCPC 34, and a transmission temperature sensor 70 sends signalsindicative of the temperature of the transmission to controller 60 foranalysis by routine 72. Controller 60 issues control signals to valves78 as a result of executing routine 76.

Routine 62 includes clutch control logic configured to receive signalsindicative of clutch commands or requests (i.e. a request to shift fromone gear to another) and determines which clutch to apply and whichclutch to release in order to execute the shift command. Such clutchcontrol logic generally includes pressure profile routines that areselectively established based on the requested or commanded shift. Eachpressure profile routine includes a plurality of pressure values thatare applied during the shift to smoothly engage and disengage theappropriate clutches. Different pressure profile may be established fordifferent shifts, i.e. the pressure profile for a shift from first tosecond gear may be different than the pressure profile for a shift fromthird to fourth gear.

Routine 64 receives outputs from routine 62, i.e., a clutch indicator,such as a clutch numbers identifying the clutche(s) to be applied orreleased, for example, and determines the pressure profile required toaccomplish the application or release of the appropriate clutches.Routine 72 determines the amount of electrical input (i.e., current)required to be sent to the clutch control valve 78 to achieve the clutchpressures required to execute the commanded or requested shift.

The amount of electrical input (i.e., current) required is a function ofthe clutch pressure required to accomplish the requested shift, thetransmission temperature, the solenoid specifications, and otherparameters that are not directly relevant to the present invention. Inthe illustrated embodiment, a look-up table is used to determine therequired electrical input based on the required pressure value receivedfrom routine 64 and the temperature value received from sensor 70. Thelook-up table values are generally based on valve specificationinformation provided by the control valve manufacturer and/ortransmission manufacturer.

Routines 66 and 74 execute portions of one or more of the pressurecontrol methods described herein to adjust the required electrical inputvalue to account for valve-to-valve differences. Routine 76 then sendsthe adjusted electrical input (i.e., current) amount to the valve 78 andvalve 78 produces the required output pressure to control the clutch.These routines execute one of a plurality of alternative methods forpressure control, including but not limited to one or more of the threemethods described below.

These routines may also include programming logic and instructions toselect one of the plurality of available methods based on the operatingenvironment, for example, a different one of the described methods maybe used if the calibration is being performed during transmissionmanufacture, during installation of the transmission in the vehicle,during operation of the transmission in a factory or testingenvironment, or during operation of the transmission in a production orcommercial use situation. As such, programming instructions and logic toperform any or all three of the described methods may be included in CPC34 and stored in memory or other suitable storage medium accessible bycontrol 32, 60.

In one embodiment, routines 66 and/or 74 include programming logic orinstructions to execute the steps shown in FIG. 3. Step 82 is executedto identify or specify one or more reference output pressures for thecalibration control 34. The reference output pressure is the amount ofpressure required to be output by a solenoid to actuate the pressureswitch before a clutch is engaged or disengaged. At step 84, a pluralityof sequenced electrical inputs are applied to the solenoid to determinethe actual current required to actuate the switch (i.e., to determinethe current required to achieve the reference pressure). The electricalinputs are ramped up until a response is received from the pressureswitch.

Step 84 also includes measuring or determining the actual electricalinput (i.e., current) required by the particular valve to produce thereference output pressure determined at step 82. Step 86 comparesreference to actual current and determines the offset(s) between theactual measured electrical input and the pre-specified referenceelectrical input amount. Step 88 includes selectively adjusting thepre-specified reference electrical inputs based on the offset(s)determined by Step 86. In other words, the reference P/I curve for thesolenoid is modified as a result of step 88. Such modifications may bedone at selected points along the P/I curve or for points withinparticular ranges of pressures, according to one of the methodsdescribed herein. In this way, reference P/I curves may be customized or“custom fit” for the solenoids in the transmission system.

The first pressure control method may be referred to as the single(lower) point calibration method. The second method described herein maybe referred to as the dual (lower and upper) point calibration method.The third method described herein may be referred to as the modifiedsingle point calibration method.

All three methods utilize a lower calibration point located (near) thelower end of the critical operating range of the transmission system.The first and second methods also use an upper calibration point locatednearer the upper end of the critical operating range of the transmissionsystem, however, in the first method, the upper calibration point ispre-specified so that the P/I curves for a solenoid or group ofsolenoids will pass through the upper calibration point. In other words,the first method effectively assumes that all solenoids in a supply havethe same electrical input requirement at one selected pressure value(the upper calibration point) located near the upper end of thetransmission operating range. The single point calibration method isthus particularly useful when the position or location of at least aportion of the individual solenoid's P/I curve along the electricalinput (“x”) axis is fairly close to the location of the reference P/Icurve provided by the supplier. The shape of the solenoid's P/I curve(i.e., its slope profile along the pressure or “y” axis) may beinconsistent relative to the reference P/I curve.

The second method uses an upper calibration point, but does not requirethe individual solenoid P/I curves to intersect the reference P/I curveat that point. The dual point method may therefore enable use ofsolenoids with P/I curves that vary in position (location along the xaxis) or curve shape (i.e., slope angle or contour) relative to thereference P/I curve. The slope angle or curve shape/contour is modifiedby both the first method and the second method.

The third method, or modified single point method does not require anupper calibration point at all. The third method is thereforeparticularly useful when the individual solenoid P/I curves have a curveshape (i.e., slope angle or contour) that is substantially consistentand similar to the reference P/I curve shape. All three methods utilizea specially configured valve assembly including a pressure switch, todetect the actual or measured electrical input values at the referencepressures. Details of each of the methods are described below.

Table 1 summarizes and compares aspects of the three pressure controlmethods. As can be seen from Table 1, the determination of which methodmay be most appropriate for a particular application depends at least inpart on characteristics of the individual solenoid P/I curves relativeto the reference P/I curve. These characteristics may be stipulated(specified to the solenoid supplier, for example) in advance, as when anorder for a supply of solenoids is placed. Alternatively or in addition,these characteristics may be determined through calibration techniquesafter the solenoids are made or installed.

TABLE 1 Single Point Dual Point Modified Single Method Method PointMethod Suitable for solenoids with shape/slope angle shape/slope angleand location inconsistent P/I curve . . . location Requires solenoidsupplier to . . . set P/I for one high keep P/I within a wide keep P/Islope pressure point tolerance band consistent Requires in transmissionlower performance lower & upper lower performance measuring of . . .point performance points point Requires pressurizing differential no yesno spool land? Offsets may be positive or yes yes yes negative? Lowpressure offsets same as lower point same as lower point same as lowerpoint Midrange pressure offsets proportional proportional same as lowerpoint High pressure offsets none same as upper point same as lower pointCan calibrate P/I at multiple yes yes yes temperatures? Switchtransition identifies lower point upper and lower points lower point

As summarized in Table 1, each of the disclosed methods modifies thesolenoid P/I curve by providing an offset in either direction (positiveor negative) along the electrical input (“x”) axis. Additionally, thesingle and dual point methods selectively modify the shape of the P/Icurve. All three of the methods are usable at multiple operatingtemperatures.

The graph of FIG. 4 illustrates aspects of the first pressure controlmethod, referred to herein as the single point method. In the embodimentof FIG. 2, routine 66 is configured to execute this method to controlsolenoid valve output pressure in an automatic transmission system of amotor vehicle. However, the method may also be used in other similarpressure control applications.

According to the single point method, solenoid performancespecifications are provided that require the greatest P/I curve accuracyat a single pressure value near the upper end of the solenoid's criticaloperating range. In other words, point 1 is a pre-specified highcalibration point at which all solenoids in a supply have the sameoutput pressure. This upper calibration point is denoted as the firstreference point (point 1) on FIG. 4. Because the offset is zero, theactual electrical input required to produce the reference outputpressure is the same as the reference electrical input. In other words,point 1 is the first reference point and also the first performancepoint.

The pre-selected specifications are toleranced about the solid-linereference P/I curve of FIG. 4. The reference P/I curve is typicallybased on published specifications or other existing specifications for aparticular model solenoid or family of solenoids; for example, thosethat may be provided by the valve manufacturer or supplier. Thereference P/I curve may be selected or modified based upon previouslyperformed iterations of one or more of the methods described herein orconventional solenoid calibration techniques. The reference P/I curvespecifications are stored in memory in the form of a look-up table,database, or similar data structure and made available to themicroprocessor or controller 32, 60 through execution of computerprogram instructions configured to access the data structure. Thereference P/I curve is shown as a solid-line curve in the variousfigures.

In the illustrated embodiment, point 1 of FIG. 4 is the specified highcalibration point. The solenoid manufacturer or supplier will adjusteach solenoid to insure that the P/I curves of all solenoids passthrough the high calibration point at a specified calibrationtemperature. Point 1 is selected to be near the upper end of thetransmission's critical operating range. Allowable (specified) solenoidpressure error for any given current is smallest at point 1 (diminishingto near zero) and increases above and below point 1.

Once the upper calibration point and reference P/I curve are determined,then a second reference point is specified or selected. The secondreference point is represented by point 2 of FIG. 4. In the illustratedembodiment, point 2 is on the reference P/I curve (solid line) and islocated near the lower end of the transmission's critical operatingrange. Point 2 may be referred to as the “lower calibration point.” Mostindividual solenoid P/I curves will actually pass to the left or rightof this point as a result of manufacturing variation. Examples ofindividual solenoid P/I curves are shown by the dashed-line curves inthe various figures.

A pressure control apparatus such as shown in FIG. 6, described below,is set to the first characterization position shown in FIG. 6 todetermine the actual electrical input, i.e., current, required for theparticular solenoid being evaluated to generate the reference outputpressure (point 2) at the lower end of the solenoid's operating pressurerange. This actual current is represented by point 3 of FIG. 4 and maybe referred to herein as a “performance point.” Point 3 is on the actual(dashed-line) P/I curve for an individual solenoid. Points 2 and 3 areat the same pressure but differ in solenoid drive current required toproduce that pressure.

Point 3 of the first method is automatically established for eachsolenoid during transmission operation or factory test, using algorithmsexecuted by routine 66 of FIG. 2 and the pressure control apparatus 100set to the position shown in FIG. 6. The pressure control apparatus 100is activated at a predetermined solenoid pressure by designing the spoolvalve assembly 104 and the return spring 148 to prevent spool movementuntil the desired solenoid pressure is reached. In the illustratedembodiment, porting of the spool valve assembly 104 changes the amountof pressure applied to switch 110 upon slight movement of the spool 134.

In all cases, pressure is removed from switch 110 when spool 134 moves.Movement of spool 134 may be caused by application of current or lackthereof, depending on the solenoid type.

When the switch 110 is actuated (i.e. current applied or removed,depending upon whether a normally high or normally low configuration isused), the transmission controller 32, 60 is signaled to record theunique current required to achieve that pressure. The process isrepeated for each clutch control solenoid 22 in the system 8.

The actual measured current required by the solenoid 102 to produce thereference output pressure (performance point 3) is then compared to thepreviously determined reference current represented by point 2 on thereference P/I curve.

The current offset, i.e., the difference between the reference currentrecommended by the controller's reference P/I look-up table for thespecified output pressure and the actual measured current performancepoint is calculated. The offset is then selectively applied to modifythe controller's P/I lookup table 72 (effectively altering the shape ofthe reference P/I curve). In the single point method, the offset isapplied proportionally over the range of pressures between the upper andlower reference points. No current offset is applied to pressurerequests above this range. The offset is applied equally to allpressures below the lower calibration point (point 2 of FIG. 4). In thisway, selective application of the offset creates a new or modifiedreference P/I curve having a different shape than the original referenceP/I curve.

More specifically, routine 74 uses the measured difference between thereference current (pre-programmed into the controller) and the actualcurrent performance point to modify the shape of the individualsolenoid's reference P/I curve between points 1 and 2 of FIG. 4.

In operation of the transmission 14, microprocessor 60 will issue apressure request, to respond to a shift request, for example. Ifmicroprocessor 60 requests the point 2 pressure, the full amount of theoffset is added or subtracted from the point 2 reference currentdetermined in the pressure-to-signal lookup routine 72. Ifmicroprocessor 60 requests a pressure at or above point 1, no offset isapplied. If microprocessor 60 requests any pressure between points 1 and2, the offset applied to the current is “prorated”, or appliedproportionally to the requested pressure. All pressure requests belowpoint 2 receive the same (full) current offset as point 2, and allpressure requests above point 1 receive the same (zero) offset as point1. Aspects of the single point method are summarized in the first columnof Table 1 above.

It should be noted that in all of the methods, reference points andperformance points are determined at the same solenoid temperature andmay be determined at a variety of different temperatures. One of avariety of known techniques for applying temperature compensation to thesolenoid may be executed by the temperature compensation routine 72,described above.

Prior art calibration methods have altered solenoid reference P/I curvesby applying an offset in only one axis. Proportional application of theoffset according to the present invention as described herein altersboth the location and shape of the reference P/I curve to more closelymatch the individual solenoid's true P/I curve and thus compensate forvariations that are impractical to control during solenoid manufacture.

The graph of FIG. 5 illustrates aspects of the second pressure controlmethod referred to herein as the dual point calibration method. Routine66 is configured to execute this method to control solenoid valve outputpressures in an automatic transmission of a motor vehicle, either inaddition to or as an alternative to one or more of the other methodsdescribed herein. However, the method may also be used in other similarpressure control applications.

According to the dual point method, solenoid performance specificationsare selected to allow “relaxed” (i.e., within a wide tolerance band)pressure limits over the full solenoid operating range. FIG. 5 shows anillustration of such specifications, wherein unlike in FIG. 4, the upperperformance point (point 4) does not equal the upper reference point(point 1). As such, less precise, and thus less costly, solenoid modelsmay be used for clutch control in the transmission.

According to the dual point method, the upper and lower reference points1 and 2, and the reference P/I curve (solid-line) are predetermined andstored in a look-up table or similar structure. Pressure controlapparatus 100 is used to determine the current required for eachsolenoid to generate two specific solenoid pressures: one near the lowerend (point 3) and one near the upper end (point 4) of a transmissionsystem critical operating pressure range. The lower point current offsetis determined in the same way as in the first method, described above,and the lower point offset is applied to pressure requests below thelower point.

The upper point current offset is determined as described below andapplied to pressure requests above the upper point. Both offsets areproportionally applied to pressure requests between the lower and uppercalibration points. The shape of the P/I curve is thus modifiedaccordingly. This process is automatically repeated at various operatingtemperatures to customize the controller's temperature compensation datafor each clutch control solenoid in the transmission.

As shown in FIG. 5, use of the dual calibration point method shouldpermit the solenoid manufacturer to supply solenoid units with wider P/Icurve variations (the distance between the solid-line curve and thedashed-line curve) than was previously acceptable, because two referenceor target points are used. The possibly wider P/I curve variationextends roughly equidistantly on either side of the initial referenceP/I curve. As noted above, point 1 is on the controller's initialreference P/I curve and is selected to be near the upper end of thetransmission's critical clutch control pressure range. In other words,point 1 is the same as the first reference point 1 described above.Point 2 is also on the reference curve but is located at the lower endof the pressure range. In other words, point 2 is the same as the secondreference point 2 described above. Points 1 and 2 are determined byprocessor 32, 60 accessing a computerized lookup table or similarstructure in which the values corresponding to the reference P/I curveare stored.

Points 3 and 4 of FIG. 5 represent actual current values determinedusing the pressure control apparatus 100 described below. Thus, points 3and 4 lie on an individual solenoid's actual P/I curve (dashed line) andare located at the same respective pressures as points 2 and 1,respectively. Point 3 may be obtained automatically during transmissionoperation or factory test using the same method as in the lower pointapproach explained above (i.e., using the first characterizationposition of FIG. 6). Points 2 and 3 are at the same pressure but differin solenoid drive current required to produce that pressure. Point 4 isautomatically established during transmission operation or factory testby pressurizing the differential spool land 146 of the pressure controlapparatus 100 as described below. When chamber 149 is pressurized,apparatus 100 assumes the second characterization position shown in FIG.7.

In the second characterization position of FIG. 7, a known hydraulicpressure 118 is temporarily applied to the chamber 149. As a result, ahydraulic force is added to the existing spring force to more firmlypreload the spool valve 104 in the “off” position. Solenoid current isthen increased by the controller 32, 60 until solenoid pressureovercomes the total preload. The valve 104 then moves and activates thepressure switch 110 as described above. Thus, a second (upper) point onthe solenoid's P/I curve is established by the controller 32, 60 usingsignals provided by the pressure switch 110. This process is repeatedfor each clutch control solenoid in the transmission and is alsorepeated at multiple temperatures.

In a dual point system as described herein, the current offset routine74 uses the measured difference between the reference current(pre-programmed into the controller) and the actual measured currentperformance point for both the upper and lower calibration pressures tocustomize the reference P/I curve for each individual solenoid.

For example, if the microprocessor 60 is requesting the point 2 pressure(same pressure as point 3) or lower, the full lower point offset isadded to or subtracted from the point 2 current. If microprocessor 60requests a pressure above the point 1 pressure (same pressure as point4), the upper point offset is added to or subtracted from the referenceP/I curve at points above point 1. If microprocessor 60 requests anypressure in the range between points 1 and 2, the offset is appliedproportionally or “prorated” along that portion of the reference P/Icurve.

As noted above, data for each pair (i.e., upper and lower) performancepoints are determined at the same solenoid temperature. Data for allreference points are set for all temperatures during transmissiondevelopment. Additional controller software may be provided, and/or theP/I data structure(s) may be customized, to gather and manage additionalpairs of calibration points (at the same two pressures) for each clutchcontrol solenoid at other temperatures. This data may be used tocustomize the reference temperature data. This is likely to furtherimprove transmission performance.

The dual point control method provides the ability to measure a secondsolenoid performance point in real time, on-board the transmissioncontrol module, and therefore enables lower cost solenoids to be usedfor clutch control in a vehicle transmission. It also may improve theaccuracy of the controller's temperature compensation tables.

The third method, like the other methods, may be used to improve shiftquality during manufacture or factory test or first time customer use ofthe transmission, to thereby increase customer satisfaction. The thirdmethod, referred to herein as the “modified single point” method, may beexecuted by routine 66 alternatively or in addition to either or both ofthe first and second methods described above. Unlike the first andsecond methods, the third method does not require stipulation of anupper reference point. Further, unlike the first method, the thirdmethod does not require the individual solenoid P/I curves to intersectthe reference P/I curve at any point. In fact, the third method isdirected to situations where the individual solenoid P/I curves do notintersect the reference P/I curve. As such, the third method may beparticularly useful to adjust the P/I curves for individual solenoidswhere the solenoids have a substantially consistent curve shape or slopeangle relative to the reference P/I curve.

According to the third method, a performance point (point 1 of FIG. 8)is determined using pressure control apparatus 100, and then the offsetbetween the performance point and the reference point is determined. Theperformance point is determined in the same manner as the lower point ofthe first and second methods disclosed above. The third method onlycompares the actual current to the reference current at the lower point.The reference pressure is near the lower end of the critical operatingrange of the solenoid. In the illustrated embodiment, the criticalsolenoid pressure range is in the range of about 90-450 kPa and thereference pressure is represented by points 1 and 2 of FIG. 8.

A gradually increasing solenoid accuracy tolerance band is specifiedstarting at the reference pressure and extending to the upper end of theoperating pressure range. The beginning of this tolerance band islocated at the current that is actually required to produce thereference pressure as long as that current falls within the specifiedcurrent range. This current is illustrated as point 1 in FIG. 8. Thecritical pressure range is illustrated by the bracketed area of FIG. 8.In other embodiments, additional accuracy tolerance may be permittedbeyond the critical operating range.

The offset between the actual and target current (i.e., the differencebetween the current recommended by the pre-selected P/I look-up tablefor the target pressure and the measured current actually required toachieve the target pressure) is calculated and applied to provide uniquecurrent offsets for each individual pressure control solenoid.

In the illustrated embodiment of the third method, point 1 of FIG. 8 isa pressure point on a typical solenoid's P/I curve. The solenoidspecifications are set to require that the reference pressure isproduced within the allowable current range at the specified calibrationtemperature. Rather than pre-selecting a high calibration point as inmethod 1, in method 3 the reference pressure is selected to match thesolenoid output pressure at which the spool valve 104 toggles thepressure switch 110 of FIGS. 12-14 described below. This target pressureis near the transmission's critical clutch control pressure range.

Point 2 of FIG. 8 is the pressure point on the reference P/I curve(solid line) stored in the memory of the transmission controller.Typically, individual solenoid P/I curves will pass either to the leftor right of this point as a result of solenoid manufacturing variation.The dashed line represents the curve of one such solenoid.

The pressure switch 110 of FIGS. 12-14 is activated at a predeterminedsolenoid pressure by designing the spool valve 104 and the return spring148 to prevent spool movement until the desired solenoid pressure isreached. Porting of the spool valve 104 applies control pressure to theswitch 110 until solenoid pressure lifts the spool 134 from itsmechanical stop. When the switch 110 toggles, the transmissioncontroller 32, 60 is signaled to record the unique solenoid currentrequired to achieve the target pressure. This process may be repeatedfor each pressure control solenoid in the transmission 14 and may berepeated at different temperatures.

Routine 66 uses the measured difference between the reference current(point 2 of FIG. 8, pre-programmed into the controller) and the actualcurrent performance point (point 1) to offset the reference P/I curve(i.e., the preprogrammed lookup table) to closely duplicate the shape ofthe individual solenoid P/I curve of point 1 (dashed line of FIG. 8) ata new location along the “x” axis as needed. In other words, the currentoffset established as described above is applied equally at allpressures in the operating range.

As with the other methods, performance points are determined at the samesolenoid temperature as reference points, which are generally set duringthe solenoid development for all temperatures in the operating range.Temperature compensation for the solenoid is provided by the temperaturecompensation routine 72 of FIG. 2, described above. The current offsetsdescribed herein may be applied equally at all transmission operatingtemperatures, or new offsets may be established at other temperatures.With all of the disclosed methods, additional curve offsets may beimplemented by using existing adaptive algorithms to further improvesystem performance.

FIGS. 9, 10 and 11 illustrate the three steps of the second, dual point,method, usable in situations where the individual solenoid P/I curves donot intersect the reference P/I curve (and therefore have asubstantially consistent slope relative to the reference P/I curve).

FIG. 9 illustrates the step of identifying the first performance point.The first performance point is located near the lower end of theoperating range and is determined by the controller 60 and the pressurecontrol apparatus 100 executing the steps of the third method, describedabove. FIG. 10 illustrates the step of identifying the secondperformance point, which is located near the upper end of the operatingrange. The second performance point is identified by the controller 60and the pressure control apparatus 100 preloading the land 146 withcontrol main pressure according to the second method as described above.An offset is determined by comparing the performance points to thelocation of the reference P/I curve. The solenoid's reference P/I curveis then modified as shown by FIG. 11, by shifting or proportionallyoffsetting the curve along the x axis, in either direction as needed, bythe amount of the offset.

The structure of pressure control apparatus 100 will now be described.It will be understood by those skilled in the art that other similarsuitable structures may be employed to perform the steps of the methodsdescribed herein. FIG. 6 illustrates the pressure control apparatus 100configured for measuring the lower point of all of the above-describedmethods. FIG. 7 illustrates the pressure control apparatus 100configured for measurement of the upper performance point of the second(dual point) calibration methods.

FIGS. 12-14 illustrate the pressure control apparatus 100 configured forother control functions which may be performed during transmissionclutch control.

Pressure control apparatus 100 is similar to a pressure controlapparatus described in U.S. Pat. No. 6,382,248 to Long, et al. Apparatus100 includes a solenoid valve 102, a pressure regulator valve 104 and adiagnostic pressure switch 110. The solenoid valve 102 is coupled to thepressure regulator valve 104, which in turn, is coupled to the pressureswitch 110 and a transmission clutch (or friction element or other loadto be controlled) 112.

A hydraulic accumulator 106 for hydraulically filtering step changes inthe output pressure of solenoid valve 102 is also shown, however, theinclusion of accumulator 106 is considered optional.

Control module 32, 60 develops a control signal 50 for activating thesolenoid valve 102, and receives a diagnostic input from switch 110 viaappropriate electrical connections (such as insulated wiring). Thesolenoid valve 102 includes a coil 108. The control signal 50 issued bymodule 32, 60 is configured to produce a desired fluid pressure inclutch 112. A control pressure source 114 and a line pressure source 116are in fluid communication with conventional fluid supply elements suchas a pump and suitable pressure regulator valves, as indicatedschematically in FIG. 1. The line pressure may have a value in the rangeof about 150-300 pounds per square inch (psi), and the control pressureis regulated to a lower value, such as a lower value in the range ofabout 100 psi.

The solenoid valve 102 is coupled to supply passage 122, exhaust passage124 and feed passage 120. Valve 102 includes a fixed housing 126 havinga pair of ports 128 and 130. An armature is movably disposed within thehousing 126. The spool port 130 is in fluid communication with passage122. Port 130 is also couplable to control pressure feed passage 120.

Port 128 is coupled to an exhaust passage 124. The armature selectivelycouples the ports 128 and 130 to variably exhaust the fluid pressure inpilot pressure passage 122. In certain embodiments, an internal springmechanism may bias the armature to a position which couples spool ports128 and 130 so that fluid pressure in passage 122 is exhausted at zerocurrent (a “normally low” solenoid). In other embodiments, where anormally high solenoid is used, the fluid pressure is exhausted at highcurrent.

Solenoid coil 108 may be actuated or energized by electrical input, i.e.current, issued by a controller 32, 60. In the illustrated embodiment,the solenoid input is a controlled direct current. Activation of thesolenoid coil 108 produces an electromagnetic force that overcomes abias, and moves the armature to un-couple the spool ports 128 and 130.In the illustrated embodiment, activation of the coil 108 by control 32,60 results in a modulated pressure in passage 122. In other embodiments,deactivation of the coil 108 modulates pressure in passage 122. Aspectsof the present invention are configurable to be used with normally highor normally low solenoids, as noted above.

The pressure regulator valve 104 has a spool element 134 as mentionedabove. Spool element 134 has subsections 136, 138, 140 that areseparated by lands 142, 144, 146, which are spaced apart along thelongitudinal axis of spool 134. Lands 142, 144, 146 extend radiallyoutward from spool 134 to selectively engage portions of a valve bore orchamber 164. As such, land 142, spool subsection 136 and land 144cooperate to define valve a subchamber 152. Likewise, land 144, spoolsubsection 138 and land 146 cooperate to define a valve subchamber 154.

Spool element 134 is axially movable within the valve bore 164 under theinfluence of return spring 148, which is disposed in a valve subchamber149 adjacent to spool subsection 140, a pilot pressure applied to apressure control area 141 of land 142, and a feedback pressure appliedto a pressure control area 147 of land 146.

FIG. 6 depicts the first characterization position of apparatus 100,which is used to obtain the lower end performance point used in each ofthe three methods described above. In FIG. 6, solenoid 102 is actuated,so that fluid ports 128, 130 are at least partially disconnected and atleast partial fluid pressure is applied to valve head 132 via passage122. Pressure switch 110 is in fluid communication with valve subchamber152 and thereby measures the output pressure of valve 104 correspondingto the electrical input applied to solenoid 102. The electrical input tosolenoid 102 is increased until switch 110 actuates indicating that thereference output pressure is obtained at the lower end performancepoint. The resulting current value specifies point 3 of methods 1 and 2described above.

In the various figures, the different shading of fluid filled regions ofapparatus 100 denotes differences in fluid pressures. In FIG. 6, fluidin chambers 124, 168, 118, 112, 166 and 170 are at the same pressure,namely, the exhaust pressure. The exhaust pressure is in the range ofabout 0 psi. Also in FIG. 6, fluid in chambers 114, 120 is at thecontrol pressure, fluid in chambers 116, 156 is at the line pressure,and fluid in chambers 110, 152 is at the output pressure measured byswitch 110. Fluid in passage 122 is at a trim pressure, which generallyvaries in the range of about 0-110 psi.

To obtain point 4 of FIG. 5, the upper pressure value, of the secondmethod (dual calibration) described above, a second characterizationposition of apparatus 100 is used. In the second characterizationposition, shown by FIG. 7, control pressure is applied to area 147 ofland 146, further counteracting the fluid pressure applied to valve head132 by passage 122. As such, a greater trim pressure is required todownwardly displace spool 134 relative to the valve chamber 164, andtherefore, a greater amount of current be applied to solenoid 102without moving the spool 134. The current applied to solenoid 102 isincreased until the second performance point (the upper calibrationpoint) is detected by switch 110. This current value specifies point 4of the dual calibration method described above. Note that the higherpressure also results in axial displacement of accumulator 106.

As noted above, the different shading of fluid filled regions ofapparatus 100 denotes differences in fluid pressures. In FIG. 7, fluidin chambers 124, 168, 112, 166 and 170 are at the same pressure, namely,the exhaust pressure. Also in FIG. 7, fluid in chambers 114, 118, 120and 149 is at the control pressure, fluid in chambers 116, 156 is at theline pressure, and fluid in chambers 110, 152 is at the output pressuremeasured by switch 110. Fluid in passage 122 is at a trim pressure,which generally varies in the range of about 0-110 psi but is higher inthe characterization of FIG. 7 than the trim pressure in FIG. 6.

The spool element 134 may also actuated to one of three states under thecontrol of solenoid valve 102, the various states being individuallydepicted by FIGS. 12, 13 and 14 during clutch control in an automatictransmission of a motor vehicle.

FIG. 12 depicts a rest or “off” state of the spool element 134 thatoccurs when the solenoid coil 108 is deactivated, exhausting the fluidpressure in pilot pressure passage 122 via exhaust passage 124. In suchstate, the return spring 148 biases spool element 134 upward, bringingvalve head 132 into engagement with passage 122. The pressure switch110, which is coupled to the fluid chamber 152 between lands 142 and144, simply detects the control pressure since the fluid chamber 152 isin fluid communication with control pressure 114. The clutch or otherfriction element 112, which is coupled to the fluid chamber 154 betweenlands 144 and 146, is exhausted via exhaust passage 168. In the “off”state, the control 32, 60 is not performing any of the self-calibratingmethods, and thus the pressure switch 110 is deactivated, because theclutch being controlled by the solenoid is fully disengaged.

FIG. 13 depicts a clutch trim state of chamber 154 of valve 104, whichoccurs when the solenoid coil 108 is actuated. A trim pressure inpassage 122 acts on valve head 132 to partially compress the returnspring 148. Such pressure also partially strokes the accumulator 106, asshown. In such state, the spool element 134 moves downwardly in thevalve chamber (in the direction of arrow 151) and land 144 decouples thefluid chamber 154 from exhaust 168. This builds fluid pressure infriction element 112, creating a feedback pressure in passage 158, whichis coupled to friction clutch 112 via restriction or orifice 150.

The force created by the feedback pressure assists the force created byreturn spring 148, and the spool element 134 dithers to alternatelycouple and decouple the fluid chamber 154 to and from exhaust passage168, thereby regulating the fluid pressure delivered to friction element112 to a level that is proportional to the pressure in passage 122. Thisregulation of pressure to the clutch 112 is configured to smoothlyengage or disengage the clutch. When the clutch 112 is trimming, land144 unblocks exhaust 166 and connects pressure switch 110 to exhaust 165via chamber 152. This change in pressure from control pressure toexhaust pressure is detected by pressure switch 110, and the pressureswitch reports the pressure change to control 32, 60 as described above.The actual current at the time of the switch actuation is captured andused by each of the methods as described above.

FIG. 14 depicts an “on” state of the spool element 134 that occurs whensolenoid coil 108 is actuated at a very high current for normally lowsolenoids. For normally high solenoids, coil 108 is actuated by very lowor zero current. In either case, actuation of coil 108 producessufficient fluid pressure to cause spool 134 to move further downwardly(in the direction of arrow 151). Port 130 connects with control pressure114, 120, resulting in control pressure being applied to passage 122 toovercome the feedback pressure and fully compress the return spring 148.When spring 148 is fully compressed, spool member 140 comes intoengagement with passage 158 at an end of travel position 162. Suchpressure also fully strokes the accumulator 106, as shown. In suchstate, land 146 fully uncovers the line pressure passage 156, therebysupplying clutch or friction element 112 with the full line pressure.Application of the line pressure to clutch 112 engages or applies theclutch.

When the clutch or friction element is to be disengaged, theabove-described process is reversed by reducing the electrical input ofsolenoid coil 108, first to an intermediate range of electrical inputsto establish trim control, and then deactivating solenoid coil 108 toreturn to the rest or off state.

The present disclosure describes patentable subject matter withreference to certain illustrative embodiments. The drawings are providedto facilitate understanding of the disclosure, and may depict a limitednumber of elements for ease of explanation. Except as may be otherwisenoted in this disclosure, no limits on the scope of patentable subjectmatter are intended to be implied by the drawings. Variations,alternatives, and modifications to the illustrated embodiments may beincluded in the scope of protection available for the patentable subjectmatter.

1. A method, comprising: receiving at least one reference output pressure value associated with a clutch control apparatus, receiving at least one reference electrical input value related to the at least one reference output pressure value, determining an actual electrical input value corresponding to the reference output pressure value, calculating at least one offset between the reference electrical input and the actual electrical input, and applying the at least one offset to the at least one reference electrical input value.
 2. The method of claim 1, comprising selectively applying the at least one offset to the at least one reference electrical input value.
 3. The method of claim 2, comprising determining whether to apply the at least one offset based on a requested output pressure.
 4. The method of claim 2, comprising applying the at least one offset in proportion to the requested output pressure if the requested output pressure is greater than or equal to a first pressure value and less than or equal to a second pressure value.
 5. The method of claim 1, comprising selectively applying no offset to the at least one reference electrical input value.
 6. The method of claim 1, comprising selecting a method of calculating the at least one offset from a plurality of methods of calculating an offset.
 7. The method of claim 6, wherein the plurality of methods includes a method in which a first reference output pressure value is a pre-specified calibration point at which the offset is zero.
 8. The method of claim 6, wherein the plurality of methods includes a method in which a first reference output pressure value is a first pre-specified calibration point at which the offset is not zero.
 9. The method of claim 6, wherein the plurality of methods includes a method in which a first reference output pressure value is a first pre-specified calibration point at which the offset is not zero and a second reference output pressure value is a second pre-specified calibration point at which the offset is not zero.
 10. The method of claim 1, comprising determining at least one characteristic of the clutch control apparatus and selecting a method of calculating the offset based on the characteristic of the clutch control apparatus.
 11. The method of claim 10, comprising executing a calibration routine to determine the at least one characteristic of the clutch control apparatus.
 12. The method of claim 1, comprising determining at least one operating temperature of the clutch control apparatus and determining the at least one offset based on the at least one operating temperature.
 13. The method of claim 1, comprising calculating a plurality of offsets and selectively applying the plurality of offsets to a plurality of reference electrical input values.
 14. A transmission control system embodied in a vehicle, the transmission control system comprising a routine configured to execute the method of claim 1 during normal or factory-test operation of the vehicle.
 15. A system comprising a routine configured to execute the method of claim 1 during manufacture or assembly of the clutch control apparatus. 